Reduction in Local Ozone Levels in Urban São Paulo Due to a Shift from Ethanol to Gasoline Use

It has been proposed that lower NOx emission fuels such as ethanol can mitigate air pollution from vehicles burning oil-based hydrocarbons. Yet, existing modeling and laboratory studies, even those seeking to simulate the same environment, vary in their predictions of how gasoline/ethanol blends affect atmospheric pollutant concentrations, including ozone. Importantly, ambient concentrations have not been evaluated during an actual – as opposed to hypothetical – shift in fuel mix in a real-world environment. Here, we report the first such study, for the subtropical megacity of São Paulo, Brazil. We combine detailed street-hour level data on regulated pollutant concentrations, meteorology, and traffic with fuel shares from a consumer demand model to compare concentrations across subsamples that differ only in the fuel mix but are otherwise similar in meteorology, anthropogenic activity, and biogenic emissions. As the gasoline share of the bi-fuel light-duty vehicle fleet rose by 62 percentage points, we estimate a robust and statistically significant reduction of about 20% in ozone concentrations, and less precise increases in NO and CO concentrations. We propose that our “model-free” analysis potentially accounts for the interaction between anthropogenic and biogenic emissions and caution that successful strategies against ozone pollution require knowledge of the local chemistry and analysis beyond the presently monitored pollutants, most notably fine particles.